Carpal tunnel syndrome (CTS), which is a pressure induced neuropathy of the median nerve at the wrist, is a common clinical problem. The carpal tunnel is a sheath of tough connective tissue that protects and encloses a variety of structures, including the flexor tendons and the median nerve. Also within the carpal tunnel is the subsynovial connective tissue (SSCT), a specially adapted paratendon that mediates movement between the flexor tendons and the median nerve. The mechanical significance of the SSCT relates to its effect on the kinematics within the carpal tunnel and, as a framework for blood and lymph vessels, the SSCT plays a fundamental role in the nutrition of the structures embedded in it.
Studies have shown that SSCT motion characteristics and thickness differ between subjects with CTS and unaffected subjects. It is believed that an increased volume of the SSCT, especially if combined with altered transmission of tendon forces through the SSCT in the carpal tunnel, affects carpal tunnel pressure and therefore increases the likelihood of CTS.
Diagnostic ultrasonography has previously been used in confirming the diagnosis of CTS and in excluding other pathologies. Specifically, ultrasonography has been used to diagnose CTS, based on static images of nerve morphology. Static ultrasound imaging for CTS diagnosis can detect thickening and echogenicity alteration of the flexor tendons and flexor retinaculum, restricted median nerve sliding in the carpal tunnel, synovial proliferation, and flattening of the median nerve. However, static ultrasound imaging cannot assess dynamic features within the carpal tunnel, for example, tendon mechanics and pathomechanics. Thus, dynamic observations of the SSCT have traditionally required surgical exposure of the carpal tunnel and are not useful for the assessment of early changes in the SSCT in individuals affected by, or at risk for, CTS.
There are a number of modes in which ultrasound can be used to produce images of objects. For example in static, “B-scan,” ultrasound imaging, the transducer transmits a series of ultrasonic pulses as it is scanned across the object along a single axis of motion. The resulting echo signals are recorded and their amplitude is used to modulate the brightness of pixels on a display. The location of the transducer and the time delay of the received echo signals locates the pixels to be illuminated. With this static method, enough data are acquired from which a two-dimensional image of the refractors can be reconstructed. Rather than physically moving the transducer over the subject to perform a scan it is more common to employ an array of transducer elements and electronically move an ultrasonic beam over a region in the subject.
Another example is Doppler ultrasound imaging. Doppler systems employ an ultrasonic beam to measure the velocity of moving reflectors, such as flowing blood cells. Blood velocity is detected by measuring the Doppler shifts in frequency imparted to ultrasound by reflection from moving red blood cells. Accuracy in detecting the Doppler shift at a particular point in the bloodstream depends on defining a small sample volume at the required location and then processing the echoes to extract the Doppler shifted frequencies.
A Doppler system is incorporated in a real time scanning imaging system. The system provides electronic steering and focusing of a single acoustic beam and enables small volumes to be illuminated anywhere in the field of view of the instrument, whose locations can be visually identified on a two-dimensional B-scan image. A Fourier transform processor faithfully computes the Doppler spectrum backscattered from the sampled volumes, and by averaging the spectral components the mean frequency shift can be obtained. Typically the calculated blood velocity is used to color code pixels in the B-scan image.
Doppler imaging has been attempted to be used for assessing tendon velocity and excursion for hand and wrist motions. However, tissue Doppler imaging is a one-dimensional method that can only quantify the axial component of motion in an angle dependent manner. Doppler measurements lose its validity when the angle between the ultrasonic beam and the tissue exceeds a certain range. As a result, static ultrasonography and tissue Doppler imaging cannot adequately assess the condition of the SSCT and a subject's risk of developing CTS.
It would therefore be desirable to develop a system and method for non-invasively analyzing the carpal tunnel, and the SSCT in particular, that could be used to generate risk factors indicative of a subject's risk of developing carpel tunnel syndrome or SSCT damage.